Hydrocarbon resources such as petroleum or natural gas have come to be produced by excavation through wells (oil wells or gas wells, also collectively called “wells”) having a porous and permeable subterranean formation. As energy consumption increases, deeper wells are being drilled, reaching depths greater than 9000 m worldwide and greater than 6000 m in Japan. In wells that are continuously excavated, the productive layer is stimulated in order to continuously excavate hydrocarbon resources efficiently from subterranean formations of which permeability has decreased over time and subterranean formations of which permeability is insufficient from the beginning. Known stimulation methods include acid treatment and fracturing (Patent Document 1). Acid treatment is a method in which the permeability of the productive layer is increased by injecting a mixture of strong acids such as hydrochloric acid or hydrogen fluoride into the productive layer and dissolving the reaction components of bedrock (carbonates, clay minerals, silicates, and the like). However, various problems that accompany the use of strong acids have been identified, and increased costs, including various countermeasures, have also been pointed out. Thus, methods of forming fractures in the productive layer using fluid pressure (also called “fracturing” or “hydraulic fracturing”) have received attention.
Hydraulic fracturing is a method in which fractures are generated in the productive layer by fluid pressure such as water pressure (also simply called “hydraulic pressure” hereinafter). Generally, a vertical hole is drilled, and then the vertical hole is curved and a horizontal hole is drilled in a subterranean formation several thousand meters underground. Fracturing fluid is then fed into these boreholes (meaning holes provided for forming a well, also called “downholes”) at high pressure, and fractures are produced by the hydraulic pressure in the deep subterranean productive layer (layer that produces the hydrocarbon resource such as petroleum or natural gas), and the productive layer is thereby stimulated in order to extract the hydrocarbon resource through the fractures. The efficacy of hydraulic fracturing has also been examined for the development of unconventional resources such as so-called shale oil (oil that matures in shale) and shale gas.
Fractures formed by fluid pressure such as water pressure immediately close due to formation pressure when the hydraulic pressure is no longer applied. To prevent a fracture from closing, a proppant is included in the fracturing fluid (that is, the well treatment fluid used in fracturing), which is fed into the borehole, thereby distributing the proppant in the fracture. Inorganic or organic materials are used as proppants included in fracturing fluid, but silica, alumina, and other inorganic particles have been conventionally used, and sand particles such as 20/40-mesh sand have been widely used because they are capable of preventing fracture closure in a very deep subterranean environment under high-temperature and high-pressure for a long time.
Various types of water-based, oil-based, and emulsion-based fluids are used as well treatment fluids such as fracturing fluid. Because the well treatment fluid must have the function of transporting the proppant to the location where the fracture is generated in the borehole, it generally must have a prescribed viscosity, good proppant dispersibility, ease of after-treatment, and low environmental load. Furthermore, fracturing fluid sometimes contains a channelant in order to form flow paths through which shale oil, shale gas, and the like can pass among the proppant. Accordingly, in addition to the proppant, various additives are used in well treatment fluid, such as channelants, gelling agents, antiscale agents, acids for dissolving rock and the like, friction-reducing agents, and the like.
The following method is typically used to produce fractures by hydraulic pressure in the productive layer of a deep subterranean formation (layer that produces the hydrocarbon resource such a petroleum such as shale oil or natural gas such as shale gas) using a fluid. Specifically, a prescribed section of a borehole (downhole) drilled in a subterranean formation several thousand meters deep is partially plugged while isolating sequentially from the tip portion of the borehole, and fracturing is performed by feeding fracturing fluid in at high pressure into the plugged section to produce fractures in the productive layer. Then, the next prescribed section (typically ahead of the preceding section, i.e., a segment closer to the ground surface) is plugged, and fracturing is performed. After that, this process is repeated until the required isolation and fracturing have been completed.
Stimulation of the productive layer by fracturing is sometimes also performed again not only for the drilling of new wells, but also for desired sections of boreholes that have already been formed. In this case as well, the operations of borehole plugging, fracturing, and the like are similarly repeated. Additionally, there are also cases where, to perform finishing of the well, the borehole is plugged to block fluid from below, and after finishing of the top portions thereof is performed, the plugging is released.
Various methods are known for plugging and fracturing of boreholes, and Patent Documents 2 and 3, for example, disclose plugs that can plug or fix a borehole (also called a “frac plug,” “bridge plug,” “packer,” or the like).
Patent Document 2 discloses a downhole plug for well drilling (also called a “plug for well drilling” or simply a“plug” hereinafter), and specifically discloses a plug comprising a mandrel (main body) having a hollow part in the axial direction, a ring or annular member along the axial direction on the outer circumferential surface orthogonal to the axial direction of the mandrel, a first conical member and slip, a malleable element formed from an elastomer, rubber, or the like, a second conical member and slip, and an anti-rotation feature. The plugging of the borehole by this plug for well drilling is performed as follows. Specifically, by moving the mandrel in the axial direction thereof, as the gap between the ring or annular member and the anti-rotation feature gets smaller, the slip contacts the slanted face of the conical member, and by proceeding along the conical member, it moves so as to expand radially, and the tip of the slip then contacts the inside wall of the borehole and is fixed in the borehole to seal the borehole, while the malleable element deforms by diametric expansion, contacts the inside wall of the borehole, and plugs the borehole as the distance in the axial direction of the mandrel decreases. It is described that metal materials (aluminum, steel, stainless steel, and the like), fibers, wood, composite materials, plastics, and the like are widely exemplified as materials that form plugs, and that composite materials containing a reinforcing material such as carbon fibers, especially polymer composite materials of epoxy resin, phenol resin, and the like, are preferred, and that the mandrel is formed from aluminum or a composite material.
Borehole plugs are arranged sequentially inside the borehole until the borehole is completed, but at the stage when the production of petroleum such as shale oil or natural gas such as shale gas (also collectively called “petroleum or natural gas” hereafter) is begun, it is necessary to release the plugging of the borehole by the slip and the diametrically expandable annular rubber member, which are members of the plug for well drilling, and to remove the plug. Because the plug is typically not designed to be retrievable after use and the release of plugging, it is removed by destruction or by making it into small fragments by pulverization, perforation, or another method, but substantial cost and time are required for pulverization, perforation, and the like. There are also plugs specially designed to be retrievable after use (retrievable plugs), but since plugs are placed deep underground, substantial cost and time are required to retrieve all of them.
Patent Document 3 discloses a disposable downhole tool (meaning a downhole plug or the like) or a member thereof containing a biodegradable material that degrades when exposed to the environment inside a well, and as the biodegradable material, discloses a degradable polymer such as an aliphatic polyester such as polylactic acid. Additionally, Patent Document 3 describes a combination of a tubular body element having an axial-direction flow bore, a packer element assembly comprising an upper sealing element, a center sealing element, and a lower sealing element along the axial direction on the outer circumferential surface orthogonal to the axial direction of the tubular body element, a slip, and a mechanical slip body. Furthermore, Patent Document 3 discloses that fluid flow in only one direction is allowed due to the fact that a ball is set in the flow bore of the cylindrical body part. However, Patent Document 3 does not disclose whether a material containing a biodegradable material is used for a downhole tool or any member thereof.
Due to increased demand for the securement of energy resources, environmental protection, and the like, particularly as excavation of unconventional resources expands, excavation conditions are becoming increasingly harsh, such as increased depth. There is a demand for a plug for well drilling with which borehole plugging and fracturing can be performed reliably, and with which the cost of well drilling can be reduced and the process can be shortened by facilitating the removal of the plug or the securement of a flow path. Therefore, various attempts have been made by trial and error within the limitations of the functional material of the plug for well drilling with regard to the selection or compositional optimization of materials having optimal mechanical characteristics or the like, to optimization of the shape such as thickness of the member forming the plugs, and to combinations of members, and the like, but a plug which sufficiently satisfies these requirements has not yet been found.